Korean war
The Korean War was caused by the division of North and South Korea. North Korea was a communist country and the South was actually American occupied. The war initially began on June 25, 1950 when North Koreans crossed the 38th parallel, the boundary between the two sides. Soon America joined the war on South Koreans side, but to America this wasn't just a war helping the South Koreans, but rather a war against Communism. In July 1953 the war ceased, but North Korea and South Korea are still separate today
http://www.history.com/topics/korean-war
http://www.history.com/topics/korean-war